Silicon carbide (SiC) is known to have a variety of crystal structure types such as cubic (so-called β-type) and hexagonal (so-called α-type) and to exhibit a wide bandgap (Non-Patent Document 1).
For example, a crystal substrate composed of cubic β-type silicon carbide has a wide bandgap and is transparent with respect to light emitted from a UV semiconductor layer. Therefore, the substrate is employed as a transparent crystal substrate (see Patent Document 1).
A UV semiconductor laser having a crystal substrate composed of cubic β-type silicon carbide and component layers such as a light-emitting layer formed on the substrate can emit light through the top surface as well as through a side surface or a backside surface, leading to excellent light extraction efficiency.
Hexagonal α-type silicon carbide (SiC) represented by a Ramsdell symbol of 2H (wurtzite), 4H, or 6H is employed as a substrate material for forming a compound semiconductor light-emitting device such as a gallium nitride (GaN) light-emitting diode (hereinafter may be referred to as LED).
For example, there is disclosed a blue-light-emitting device having an α-type silicon carbide crystal substrate with a {0001} crystal plane as a surface, and a light-emitting layer composed of gallium indium nitride (GaYInZN: 0≦Y, Z≦1, Y+Z=1) deposited on the surface (see Patent Document 2).
Silicon carbide (SiC) is also employed as a substrate material. For example, a compound semiconductor light-emitting device having a grown layer deposited on a silicon carbide (SiC) single-crystal substrate is disclosed (see Patent Document 3).
Meanwhile, hexagonal silicon carbide (SiC) has an a-axis lattice constant (a) of 0.308 nm, which is almost equivalent to the a-axis lattice constant (a) of gallium nitride (GaN) (0.319 nm). Therefore, hexagonal silicon carbide (SiC) and gallium nitride (GaN) can be joined together with few misfit dislocations.
A pn-junction light-emitting diode based on the lattice matching property is disclosed, the diode having a hetero-junction structure including a p-type silicon carbide (SiC) layer and an n-type hexagonal aluminum gallium nitride (compositional formula: AlXGaYN: 0≦X, Y≦1, X+Y=1) layer (see Patent Document 4).
A blue-light-emitting device having a junction structure between a p-type silicon carbide (SiC) single crystal and an n-type silicon carbide (SiC) single crystal is also disclosed (see Patent Document 5).
As described above, some cases where a light-emitting layer composed of silicon carbide (SiC) is used have been disclosed. However, in general, silicon carbide (SiC) serves as a substrate material, and a compound semiconductor device having grown layers deposited on a silicon carbide (SiC) crystal substrate as disclosed in Patent Documents 1 to 3 is employed in a variety of fields.
When a silicon carbide (SiC) crystal substrate is employed, component layers including a light-emitting layer are not directly formed in the crystal substrate. There are disclosed methods for forming a light-emitting device including forming a boron phosphide (BP) buffer layer on a silicon carbide (SiC) crystal substrate, followed by formation of component layers including a light-emitting layer (see Patent Documents 6 and 7).
Another compound semiconductor light-emitting device is disclosed in which a super-lattice layer including a boron phosphide (BP) buffer layer is formed on a silicon carbide (SiC) crystal substrate (see Patent Document 8).
[Non-Patent Document 1]
Y. Kumashiro, Electric Refractory Materials, (USA), Marcel Dekker Inc., (2000), p. 409-411
[Patent Document 1]
Japanese Patent Application Laid-Open (kokai) No. 4-84486
[Patent Document 2]
Japanese Patent Publication (kokoku) No. 55-3834
[Patent Document 3]
Japanese Patent Application Laid-Open (kokai) No. 60-207332
[Patent Document 4]
Japanese Patent Application Laid-Open (kokai) No. 2-177577
[Patent Document 5]
Japanese Patent Application Laid-Open (kokai) No. 2-46779
[Patent Document 6]
Japanese Patent Application Laid-Open (kokai) No. 2-275682
[Patent Document 7]
Japanese Patent Application Laid-Open (kokai) No. 2-288388
[Patent Document 8]
Japanese Patent Application Laid-Open (kokai) No. 2-288371